1. Filed of the Invention
This invention relates to methods and apparatus for separating emulsions; in particular, this invention relates to methods and apparatus for separating water-in-oil emulsions; and most particularly, this invention relates to methods and apparatus for separating water-in-oil invert drilling fluids.
2. State of the Art
In drilling operations, for example, drilling operations to recover petroleum, drilling fluids pumped down a drill string remove rock cuttings produced by the drill bit from the borehole to the surface. The drilling fluid also helps to control subsurface pressures and provides a protective and stabilizing coating to permeable formations.
Drilling fluid is pumped through a hollow drill string and the drill bit into the borehole while the well is being drilled, thereby cooling and lubricating the drill bit and the drill string. The fluid is then forced up the borehole and through the annulus between the drill string and the wall of the borehole to the surface. At the surface, the rock cuttings are filtered from the drilling fluid through a shaker screen and the screened fluid is re-circulated to the borehole through the drill string and drill bit.
The driller constantly monitors and adjusts the consistency and properties of the drilling fluid during the operation, for example, to compensate for pressure changes within the well as the drill bit penetrates the various rock strata.
Specific drilling fluid systems are selected to optimize a drilling operation in accordance with the characteristics of a particular geological formation. Those skilled in the art refer to drilling fluids comprising liquid, usually water or oil, and solids in suspension as a drilling mud. Oil-based drilling muds usually contain oil as the continuous phase, although frequently a separate water phase is emulsified in and dispersed throughout the oil phase so that there is no distinct or separate layer of water in the mud. Such water-in-oil drilling muds or fluids will herein be referred to as water-in-oil invert drilling fluids. Water-in-oil invert drilling fluids are normally used to drill through swelling or sloughing shales, salt, gypsum, anhydrite or other evaporite formations, hydrogen sulfide-containing formations and to drill holes through hot (&gt;300 degree F.) formations.
Water-in-oil invert drilling fluids are far superior to the water-based systems for sectors of off-shore drilling and for penetration of water-sensitive layers. They are preparations of the type of water-in-oil emulsions, that is the aqueous phase has broken up into small particles which are heterogeneously distributed in a finely dispersed state within a continuous oil phase. The drilling fluid will contain a solid component, usually a suitable clay. For stabilizing the system altogether and for establishing the desired performance properties, a multiplicity of additives is provided, more specifically emulsifiers or emulsifier systems, weighting agents, fluid-loss additives, alkali reserves, viscosity modifiers and the like.
All water-in-oil invert drilling fluids are designed to function with a range of water contents. This is a necessary feature, since water can enter the mud as a contaminate at any time as one drills though the geologic formations. Normally, the mud will be maintained with a lower water content and a higher mud weight.
Conventionally, the oil phase of such a mud is No. 2 diesel oil, but other oils can be used in special situations, such as a non-polluting mineral oil in environmentally-safe drilling fluids. The water phase may range from freshwater (defined herein to be any water containing less than 1% total dissolved solids) to near saturated calcium chloride water. However, it is never desirable to allow the water to become saturated or super-saturated with calcium chloride. In this condition, a hydrate of the salt will come out of solution as the mud cools and carry water that will weaken the emulsion and cause water wetting of the mud solids. From 35 to 38 percent calcium chloride in the water in an upper limit for safe operation.
Many water-in-oil invert drilling fluids use a calcium or magnesium fatty-acid soap as the primary emulsifier. Such a soap adequately emulsifies the mud at temperatures up to about 275 degree to 300 degree F. for freshwater or sodium chloride water. At higher temperatures and for calcium chloride water, special supplemental emulsifiers, generally polyamides, are needed. The soap also adds viscosity to the oil and provides a weak gel structure which helps in barite suspension.
The emulsion adds viscosity and enhances fluid loss control to the mud compared to non-emulsified muds. The emulsified water droplets affect fluid viscosity in the same manner as inert solids. As mud density is increased, it is necessary to decrease the maximum allowable water content in order to minimize the plastic viscosity. At mud weights above 18 lb/gal. the water content should be less than 12 percent. The water also decreases fluid loss in the same manner as oil emulsified in a water-base mud.
All of the solids in an oil-mud must be wet by the oil to prevent agglomeration that causes high viscosities and settling of the particulates. Since barite and drilled solids are naturally water-wet, an oil-wetting agent is necessary in oil-mud. The soaps 15 do some of the oil wetting. However, they are not strong enough nor do they act fast enough to handle a large influx of water-wet solids. Rapid additions of barite, fast drilling in soft shales, and water-mud contamination are all cases where a special oil-wetting surfactant is needed.
Although the soap and water contents provide viscosity to an oil-mud, additional viscosity is often needed for suspension, especially in the lower mud weight range.
Either asphalts or amine treated bentonite are normally used for this purpose. Asphalt either softens or goes into solution in the oil to cause thickening of the oil. It may also react with other mud components to cause development of a grease-like structure. The amine treated bentonite is dispersible in oil and acts as a colloid to cause increased viscosity.
Despite the advantages of water-in-oil invert drilling fluids they have one tremendous disadvantage--once they have been used of a drilling operation, they contain the additives for that particular hole and are not useful for recycling, so the driller has a large quantity of muddy emulsified oil that is are difficult to dispose of. Drilling a borehole produces drill cuttings, comprised of pulverized rock and invert mud residues, which are a muddy water-in-oil emulsion, as a waste material. As the use of water-in-oil invert drilling fluids has increased in the last several years due to drilling deeper holes, there has been a substantial effort towards a reduction of the environmental impact of oil-contaminated drill cuttings. After the well has been drilled there is a large quantity of a muddy water-in-oil emulsion containing any number of additives and enhancers, as well as all the drill cuttings, that is, the fragments of rock formed by the drill bit, and removed by to conveying upwardly along with the drilling fluid. In practice, these amounts of rock cuttings produced are separated by one or more steps of sieving and additional separating steps such as centrifugation from the major amount of the recycled drilling fluid phase. But the cuttings still have a significant amount of oil on them. All of these waste products should be disposed of in an environmentally friendly manner.
One technique suggested for the treatment and disposal of invert oil waste has been the use of land-farming. Land-farming is a natural waste management process with a minimal energy input requirement. A land-farming site is prepared by stripping and stockpiling topsoil and a layer of humus from the site. The drill cuttings are spread over the area and covered with the stockpiled topsoil and humus. The oil-contaminated drill cuttings and soil are then cultivated and mixed, thereby increasing the contact of drill cuttings with indigenous microorganisms in the soil for microbial degradation of the associated oil. Nitrogen fertilizers are added to enhance the activity of the microorganisms, hereinafter, the active agents. Optionally, a bacterial culture containing active agents, such as manure, can be added to the site.
The cost of land-farming is relatively low compared to other disposal methods.
Moreover, this method does not cause the air emission problems of smoke and particulate matter which may be of concern in incineration facilities. The practice of land-farming has gained considerable approval from environmental regulatory agencies as a method of dealing with hydrocarbon wastes. Another environmental and economical benefit of land-farming is that the disposal can generally be effected without transport of the drill cuttings.
However, for all the purported advantages of land-farming, the technique relies on an ill-defined biological system, and as such can be exquisitely sensitive to poisoning, death of the active agents, infection of the active agents by non-active agents, and the like, as well as to variables that may not be apparent to the operators of the land-farm. The high toxicity of the aromatic components of diesel fuel presents one of the biggest problems for land-farming of water-in-oil invert drilling fluid residue. Since oil-based drilling fluids frequently contain diesel oil fractions containing high concentrations of these toxic aromatic constituents, they can devastate land-farms meant to help in their disposal. Consequently, this form of disposal is not available to many water-in-oil invert drilling fluid waste products.
Another problem is encountered by off-shore drillers. When water-in-oil invert fluids were first introduced, they were frequently disposed of by dumping directly into the ocean. It has been found that non-biodegradable mineral oil water-in-oil invert drilling fluids and drill cuttings disposed of by dumping to the bottom of the ocean remain there for years, obviously an undesirable ecological situation. Such waste still adversely affects sensitive marine eco-system years after such unwise disposal. Moreover, the same problems will arise upon work with water-based oil-in-water emulsion fluids.
In order to allow disposal of the water-in-oil invert drilling fluids, ecologically compatible and especially of biodegradable oil phases have been suggested for use in connection with the composition of oil-containing drilling fluid systems. Non-polluting oils suggested include mineral oil fractions free of aromatics, vegetable oils, for example peanut oil, soybean oil, linseed oil, corn oil and rice oil, and oils of animal origin such as whale oil. However, subsequent, more detailed investigations have proved that the readily biodegradable oils of vegetable or animal origin cannot generally be used for practical reasons. For example, the Theological properties of such oil phases can not be controlled over the temperature range as required in practice of from low temperatures ranging from about 0 degrees C to high temperatures of 250 degree. C and higher temperatures that are routinely encountered in actual practice.
So the problem of disposing of the used water-in-oil invert drilling fluid in an environmentally acceptable way continues to present itself. The preferred practice used is simply the storage of the muddy water-in-oil emulsion containing all the drilling additives and fine cuttings in a hope that some day it can be processed. Millions of tons of used water-in-oil invert drilling fluid are stored at various locations around the world. Even if all the water-in-oil invert drilling fluids of the world were somehow made entirely environmentally satisfactory today, the continuing problem of stored used water-in-oil invert drilling fluids must still be addressed.
A variety of separation techniques are known for separating intractable emulsions. For example, Konoijn has invented several apparatus and methods for separating liquids from gases, see for example, U.S. Pat. No. 5,683,629 and 5,300,132, but the techniques shown therein are not generally applicable to the distinct task of separating liquids from immiscible liquids when an emulsion is present.
Other attempts have also been made to separate sand and soil from a combination of water and oil for remediation of contaminated sites. Too, for example, in U.S. Pat. No. 5,344,255, teaches a method of using a surfactant to remediate sites contaminated with oil. However, the emulsion he is faced with is less concentrated and less intractable than the emulsions used for water-in-oil invert drilling fluids, which are fluids made specially to be emulsions that will not separate under the extreme conditions found down-hole.